Pigments have been used as paint or printing ink, and recently as a coloring agent for a color filter resist or an inkjet ink. Pigments have excellent fastnesses of various properties such as heat resistance, weather resistance and migration resistance, for example as compared to dyes. However, when the pigments are dispersed in a composition, they present potential problems such as increased composition viscosity after pigment dispersion, increased aggregation or precipitation in a composition, increased composition viscosity over time, and color flooding when heterogeneous pigments are mixed.
Recently, with the demands for high contrast liquid crystal displays, fine imaging devices, and high coloring/high sharpness inkjet ink, the demands for making micro-particle pigments and high concentration pigments in the dispersion compositions have greatly increased. However, as particle diameters become finer and concentrations of pigments increase, particles are prone to aggregation, thus making it difficult to obtain a stable dispersion composition.
In order to solve these problems, improvement of a pigment itself (surface treatment of pigment), development of dispersants and surfactants having good adsorption with pigments, and use of pigment dispersants have been proposed. Pigments used in a color filter or a inkjet ink may include phthalocyanine-based pigments, diketopyrrolopyrrole-based pigments, anthraquinone-based pigments, quinacridone-based pigments, dioxazine-based pigments, isoindoline-based pigments, isoindolinone-based pigments, azo-based pigments, and dye lake pigments. And a large number of pigment dispersants, in particular phthalocyanine-based pigments, anthraquinone-based pigments, and quinacridone-based pigments have been disclosed. For example, a method for mixing sulfonated products of a pigment or metal salts thereof with pigments (Patent Documents 1 to 3), a method for mixing substituted amino methyl derivatives (Patent Document 4), and a method for mixing phthalimide methyl derivatives (Patent Document 5) are known in the art.
The methods are effective for pigments with certain chemical structure, but are not useful for pigments with chemical structure, into which sulfone groups, amino methyl groups, phthalimide methyl groups, and the like are difficult to be introduced. In particular, because an appropriate pigment dispersant has not been provided for C.I. Pigment Yellow 150, which is commonly used in color filter and inkjet fields, the pigment is very difficult to disperse. In Patent Documents 6 and 7, an inclusion compound, in particular, a melamine compound is added into C.I. Pigment Yellow 150 to maintain dispersion stability, but the effects may not be obtained unless the melamine compound is added in an amount equal to or more than the pigment and the demand for high concentration pigments in the dispersion composition may not be satisfied.
Patent Document 1: Japanese Patent Application Publication (JP-B) No. 41-2466
Patent Document 2: Japanese Patent Application Laid-Open (JP-A) No. 63-172772
Patent Document 3: Japanese Patent Application Publication (JP-B) No. 50-4019
Patent Document 4: Japanese Patent Application Publication (JP-B) No. 39-16787
Patent Document 5: Japanese Patent Application Laid-Open (JP-A) No. 55-108466
Patent Document 6: Japanese Patent Application Laid-Open (JP-A) No. 2000-119544
Patent Document 7: Japanese Patent Application Laid-Open (JP-A) No. 2005-344055